The invention relates to marine drives, and more particularly to a method and apparatus to improve the reverse thrust in a reversible marine drive.
It is common for marine drives to be equipped with "through-hub" exhaust passages to provide an underwater exhaust passage which not only muffles the engine exhaust noise, but also directs the engine exhaust away from the propeller preventing disturbance of the solid flow of water into the propeller when in the forward direction. However, when in the reverse direction, the rearward discharging engine exhaust is directed back into the blades of the propeller as the propeller hub and marine drive move rearward. This action creates excessive turbulence of the water flow into the propeller which reduces the efficiency, the effectiveness, and the reverse thrust of the propeller and increases cavitation, ventilation and erosion of the propeller blades.
Reduced reverse thrust has been a long standing problem in the industry and several attempts have been made to increase the reverse thrust but have either created increased exhaust noise, been uneconomical to produce, been ineffective, or not been adaptable to retrofitting existing marine drives.
One solution to this problem, generally used in sailboat drives, is to provide a secondary exhaust passage above the propeller hub to discharge exhaust in the reverse direction away from the propeller. However, this creates excessive noise because of the proximity of the passage to the water surface and is not readily adaptable to existing marine drives.
Several prior art patents use specially manufactured propeller hubs and components, such as spring biasing and mechanized shifting devices, to slide the propeller hub fore and aft providing a second exhaust passage forward of the propeller. Examples include U.S. Pat. No. 3,871,324 issued Mar. 18, 1975 to Snyder, U.S. Pat. No. 3,754,837 issued Aug. 28, 1973 to Shimanckas, U.S. Pat. No. 3,556,041 issued Jan. 19, 1971 to Shimanckas, and U.S. Pat. No. 3,467,051 issued Sep. 16, 1969 to Shimanckas, none of which are readily and economically adaptable for retrofitting existing marine drives and none of which use unmodified existing propeller hubs.
Another attempt was made by U.S. Pat. No. 4,023,353 issued May 17, 1977 to Hall, which uses an automatically actuated butterfly valve to change the direction of the exhaust flow depending upon the direction of the marine drive, again, not readily adaptable to existing marine drives and requiring a specially manufactured hub and components.
Other examples not having a slidable propeller hub, but yet attempting to increase reverse thrust, include U.S. Pat. No. 4,778,419 issued Oct. 18, 1988 to Bolle, et al. which provides an opening forward of the propeller hub and uses the water entering the rear of the propeller hub to force exhaust out the forward opening. Others have used similar techniques and/or incorporated outer shrouds, or the like, including U.S. Pat. No. 4,511,339 issued Apr. 16, 1985 to Kasschau, U.S. Pat. No. 4,436,514 issued Mar. 13, 1984 to Takahashi et al, U.S. Pat. No. 4,388,070 issued Jun. 14, 1983 to Kasschau, U.S. Pat. No. 4,276,036 issued Jun. 30, 1981 to Nishida et al. These devices do not have slidable propeller hubs, nor do they block rear discharge, but rely upon a sufficient amount of water flow into the rear passage to prevent exhaust discharge therethrough. The success of these devices therefore is at least partially dependent upon reverse speed. However, several of the applications requiring increased reverse thrust typically operate at slower speeds in the reverse direction. For example, it is desirable in fishing and sailing boats to move rearward slowly, yet it is desirable to produce increased reverse thrust which allows the engine to operate at lower speeds. It is also desirable for the same boats to operate at high reverse speeds in which increased reverse thrust would also be beneficial.